DESCRIPTION: (Verbatim from application) The objective of this grant proposal is to examine the roles of satellite cells and the calcineurin-NFAT pathway in regulating muscle fiber size under two different physiological states: functional overload and recovery from atrophy. Some investigators have proposed that the cell volume:myonuclei ratio remains constant irrespective of the physiological state. According to this concept, muscle fiber growth during functional overload or recovery from atrophy can only occur by activating satellite cells and producing more myonuclei. In contrast, other investigators have stressed that regulation of skeletal muscle fiber size is mediated by many interrelated signaling pathways that ultimately activate transcriptional events, or affect the existing products of gene transcripts. However, the role of NFAT transcription elements in mediating skeletal muscle hypertrophy is not completely understood, and conflicting results have been produced from tissue culture, in vivo, and transgenic studies. I will more rigorously test the importance of the NFAT pathway and satellite cells for muscle hypertrophy and functional rehabilitation, and will add more relevant techniques to existing studies. Following either hind-limb suspension or compensatory overload, muscle will be assayed for NFAT expression and subcellular localization, and the activity of calcineurin, a key phosphatase in the pathway, will be determined. Additional manipulations involving the destruction of satellite cells through irradiation will further elucidate the importance of existing structural elements in the muscle.